1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium suitable for high density magnetic recording, and a magnetic storage apparatus using the same.
2. Description of the Related Art
Along with the rapid propagation of personal computers, work stations, and the like, there has been a growing demand for increasing the capacity of a magnetic  disk apparatus, which is the core of a non-volatile file system. In order to increase the capacity of a magnetic disk apparatus, it is necessary to increase the recording bit density, i.e., the plane recording density. A recording system employed for currently-used magnetic disk apparatuses is called “in-plane recording system”. In this system, a ferromagnetic layer having a large coercivity in a direction parallel to the disk substrate plane is used as a recording medium, and information is recorded by magnetizing the recording medium in a direction in the substrate plane. In such a case, bit “1” corresponds to a magnetic inversion section where two adjacent portions are oppositely magnetized to each other, i.e., at an angle of 180°. In order to increase the in-plane recording density, it is necessary to increase both the bit density in the circumferential direction of the disk (linear recording density) and the bit density in the radial direction (track density). While the track density is limited by the track width formation process or the positioning precision of the read/recording head, they are primarily only technical problems. It is believed that the linear recording density, on the other hand, is subject to a limitation in principle due to the characteristics of the recording medium.
In an in-plane recording system, two oppositely-magnetized portions exist facing each other with a magnetic inversion point therebetween, thereby creating a large demagnetizing field. Due to the demagnetizing field, a transitional region of a finite width is formed in the magnetic inversion section. The width of the magnetic inversion region needs to be at least smaller than the bit interval. Accordingly, in order to increase the linear recording density, it is necessary that the medium is magnetized despite the demagnetizing field. More specifically, it is necessary to reduce the thickness of the recording magnetic layer while improving the coercivity of the medium. Thus, the linear recording density is greatly limited by the magnetic properties of the medium. In a standard in-plane magnetic recording system, it is desirable that the ratio of the linear recording density with respect to the track density is about 10. In order to realize a recording density of 50 Gb/in2 under such a condition, the bit interval in the circumferential direction is about 25 nm. A magnetic properties estimation with a simple model shows that a medium in which the width of the magnetic inversion region is 25 nm or less needs to have a thickness of 15 nm or less and a coercivity of 5 kOe or more.
However, with a coercivity over 5 kOe, it is difficult to ensure a recording magnetic field with which the medium can be sufficiently magnetized. Moreover, with a Co-alloy-based magnetic layer, when the thickness of the magnetic layer is 15 nm or less, the substantial volume of the medium crystal grain is so small that the magnitude of the thermal energy of the grain is non-negligible with respect to the magnetic anisotropy energy thereof. As a result, the influence of the thermal fluctuation becomes significant, thereby posing a problem of thermal decay, where the magnitude of the recording magnetization decreases over time. When one attempts to ensure the crystal grain volume with the crystal size in the in-plane direction, there will be an increase in the medium noise, whereby a sufficient S/N ratio cannot be obtained. Thus, difficulties in principle are expected in realizing an in-plane recording density of 50 Gb/in2 or more while achieving a sufficient thermal decay resistance and low noise.
A perpendicular magnetic recording system is a system in which a thin-film medium is magnetized in a direction perpendicular to the film plane thereof. It is believed that a perpendicular magnetic recording medium is different from an in-plane magnetic recording medium in the prior art in terms of the recording principle and the mechanism in which medium noise occurs. In the perpendicular magnetic recording system, adjacent magnetized portions are not facing each other but are in an antiparallel arrangement, whereby there is no influence of a demagnetizing field. Therefore, it would be expected that a magnetic inversion can be achieved within a very narrow region, and it is easier to increase the linear recording density. Moreover, since the demand for reducing the thickness of the medium is not as strong as that for an in-plane recording medium, it is possible to ensure a high resistance to the thermal decay. Thus, the perpendicular magnetic recording system has been attracting public attention as a system that is essentially suitable for high density magnetic recording, and various medium materials and structures have been proposed therefor. Various perpendicular magnetic recording systems include those in which a single perpendicular magnetic layer is used and those in which a magnetic under layer is provided on a perpendicular magnetic layer. A technique using a two-layered perpendicular magnetic recording medium having a magnetic under layer is described in, for example, IEEE Transaction on Magnetics, Vol. MAG-20, No. 5, September 1984, pp. 657-662, “Perpendicular Magnetic Recording-Evolution and Future”. A medium in which a perpendicular magnetic layer made of a CoCr alloy is provided on an under layer made of a soft magnetic material such as a permalloy is a candidate for the perpendicular magnetic recording medium for this system.
In order to realize a magnetic storage apparatus capable of high density magnetic recording of 50 Gb/in2 or more based on a perpendicular magnetic recording system using a two-layered perpendicular magnetic recording medium, it is necessary to reduce the medium noise. The medium noise occurs from both the perpendicular magnetic layer and the magnetic under layer, and particularly the spike-shaped noise occurring from the magnetic under layer has been a problem. An example of such noise is described in, for example, IEEE Transaction on Magnetics, Vol. MAG-20, No. 5, September 1984, pp. 663-668, “Crucial Points in Perpendicular Recording”. In order to address such a problem, a method for forming an in-plane magnetic layer under a magnetic under layer has been proposed in the art, as seen in Journal of the Magnetics Society of Japan, Vol. 21, Supplement No. S1, pp. 104-108 “Increasing S/N of Three-Layered Perpendicular Medium and Stability of Recorded Signal”. However, it was not sufficient for realizing a magnetic storage apparatus capable of high density magnetic recording of 50 Gb/in2 or more.